A typical oil well comprises a cylindrical well casing, a concentric production tubing which is narrower in diameter than the casing and which is lowered within the casing, and a pump located at the lower end of the production tubing, which is submerged below the fluid level in the casing.
A conventional type of pump used in many wells has a barrel connected to the production tubing below the level of the oil in the well casing, a standing (inlet) valve located proximate to the bottom of the barrel, a plunger reciprocated within the barrel, and a travelling (or outlet) valve. The travelling valve is usually associated with, and actuated by, the plunger.
In operation the plunger is reciprocated, as for example by a string of sucker rods which is reciprocated from apparatus at the surface, such as a pumpjack. As the plunger is lifted, the travelling valve is forced closed, which tends to create a vacuum in the barrel beneath the plunger. This forces the standing valve open, whereby oil in the casing is drawn into the barrel. When the plunger descends, the standing valve closes, the travelling valve opens, and oil in the barrel is forced to pass through the travelling valve into the production tubing above the plunger. As the plunger is reciprocated through a series of upstrokes and downstrokes, oil within the oil well casing is forced into the production tubing, producing a column of oil in the tubing above the plunger. Eventually this column reaches the surface, whereupon each subsequent cycle of the plunger pumps a volume of oil (equal to the volume within the barrel displaced by the movement of the plunger) to the surface.
Due to the long length of the string of sucker rods, the total elasticity of the string of rods is very large. This causes stretching of the string during the upstroke, and compression or buckling of the string during the downstroke. This reduces the effective downward force acting on the plunger when it is moving downwardly in the barrel, since some of this force is absorbed within the string of rods. In many wells the lowering of the rods is further impaired by obstructions in the well and by the viscosity of oil.
Thus, in many wells, depending on such factors as the depth and vertical orientation of the well, and the viscosity of the oil being pumped, the downward force acting on the plunger during the downstroke is insufficient to force the plunger to traverse the pump barrel fully, reducing the productive capacity of the pumping system, and underutilizing the reciprocating action of the pumpjack or other surface apparatus. Further, the sucker rods can wear or become damaged from the buckling, especially if they are of glass fibre or composite material.
The fluid in the production tubing constitutes a long fluid column, often thousands of meters in height. Therefore, there is a pressure (hereafter referred to as the "tubing pressure") on the plunger (which is at the bottom of that column) from the weight of the fluid above it. However, during the downstroke in a conventional pump, this pressure acts on both sides of the plunger (because the travelling valve is open and the standing valve is closed during the downstroke). Thus there is no net downward force on the plunger arising from the weight of the fluid column during the downstroke in a conventional pump, despite the fact that the tubing pressure is greater (and often substantially greater) than the "casing pressure" (the pressure of fluid in the casing in the vicinity of the pump).
There have been various attempts to make use of the weight of the fluid in the production tubing to force the plunger downward.
U.S. Pat. No. 3,140,667 issued to Robert L. Anderson et. al. in 1964 discloses an apparatus which attempts to overcome this problem by using a subsurface pump which embodies two barrels and two plungers of the same cross section and two annular sections separated by an annular packing assembly. During the downstroke oil is drawn from the casing into one of the annular sections. On the upstroke it is forced from this annular region into the production tubing.
U.S. Pat. No. 3,918,845 issued Nov. 11, 1975 to Allen Heard and U.S. Pat. No. 3,912,240 issued Oct. 14, 1975 to Robert H. Gault both disclose pumps which have dual plungers reciprocating within dual barrels with three valves and a central packing assembly for dividing the barrel into two regions, one above, and the other below the packing. Each region is divided again by its respective plunger assembly. Both of these patents provide for fluid between the packing and the top plunger to be in communication with fluid at the casing pressure to produce a net downward force, while the oil in the lower chamber is sealed off by means of the said packing. Oil is first pumped into the region below the lower plunger during one stroke, and then into the region between the lower plunger and the packing during the next stroke before being forced into the production tubing during the subsequent stroke.
All three of these patents disclose pumps utilizing at least 3 packing seals, each of which are subject to wear. Furthermore, these pumps are complex in construction and involve a central packing assembly which complicates withdrawal of the plungers when the packing seals wear. There exists a need for an effective downhole pump of simple construction which utilizes the pressure of the fluid column to produce a net downward force during the downstroke.